During the course of drilling an oil or gas well, various operations often require a downhole electric power source. For example, a power source is required to operate circuits and instruments that measure the production capabilities of a well during the initial drilling of an oil well. In the prior art, batteries are typically used to provide the necessary power in a well.
However, a significant problem with the use of batteries as a power supply in a downhole environment is their limited life expectancy. Failure of a battery in a downhole power source can cause much delay and expense.
For example, one operation which is often performed during the course of drilling an oil or gas well is to lower a testing string into the well to test the production capabilities of hydrocarbon producing underground formations intersected by the well. Testing is typically accomplished by lowering a string of pipe, generally drill pipe or tubing, into the well with a packer attached to the string at its lower end. Once the test string is lowered to the desired final position, the packer is set to seal off the annulus between the test string and the wellbore or casing, and the underground formation is allowed to produce oil or gas through the test string. Measurements of pressure and temperature are then taken by transducers within the string, and the data is either stored in a memory, such as a random access memory (RAM) chip, within the testing circuit or transmitted to the surface. Depending on the data required for well test objectives, testing may last anywhere from one day to two weeks.
If a battery required by the downhole power source for the down hole instrument fails prior to the completion of testing, the testing string must be pulled back to the surface and the battery replaced or valuable rig time may be wasted on testing with no recorded data. This procedure causes an interruption in the collection of data as well as delay and further expense in the drilling process.
In addition, batteries can only provide a limited amount of power even when fully charged. This disadvantage limits the use of circuits requiring higher power in a downhole environment.
Accordingly, a need has arisen in the industry for a method and apparatus to extend the life of a downhole power source and/or increase the amount of power output from a downhole power source.